The inside of a furnace structure as an industrial furnace, in particular, a coke oven, a blast furnace, a steel manufacturing furnace, and the like, as the iron and steel making equipment, contacted with a molten material such as a carbonized coal, a molten iron, a molten steel, a slug, and the like, is in a severe environment exposed to a temperature as high as 1000.degree. C. or more. In particular, at the time of the coke extruding operation from a coke oven carbonizing room, or of the operation of injecting, storing, or discharging a molten iron or a molten steel in a steel manufacturing furnace, the internal wall experiences a remarkable temperature change. Therefore, in the internal wall, not only a damage by melting by the penetrated molten material but also damages including cracks and peel-off by heat spalling are frequently encountered.
In order to cope with the various damage factors, an appropriate brick material needs to be selected at the time of designing or furnace construction as well as mending is required in order to prolong wall life.
For example, as the mending technology, a flame spray mending method, where a mending material is blown thermally to a refractory damage part, can be presented. The flame spray mending method is a technology where a flame spray mending material containing a mending flame resistant oxide powder or an easily oxidizable powder, or a mixture of both, having a composition substantially the same as that of the material of the furnace wall refractory to be mended is thermally blown mainly to a high temperature furnace internal wall surface. According to the method, the flame resistant oxide powder is melted by the combustion heat of a combustible gas, and the easily oxidizable powder becomes an oxide by being melted exothermically by its own combustion so that a spray mending layer can be formed with the flame resistant oxide powder. In particular, since the furnace temperature of a coke oven cannot be lowered except the time of rebuilding and thus the furnace wall mending is done as a prerequisite in a high temperature state, such a flame spray mending method is effective.
As a conventional technology concerning such a flame spray mending method, for example, the method disclosed in the official gazette of Japanese Examined Patent Publication No. 2-45110 can be presented. The method is a dry method comprising the steps of mixing a powdery flame resistant oxide with a combustible material and a combustible gas so as to be supplied to a combustion supporting gas containing oxygen including oxygen and air for thermally melting the flame resistant oxide powder by the heat of the combustion flame and blowing the same to the damage part of the internal wall of the furnace instantaneously. It is characteristic of the method that the spray mended refractory is highly durable compared with a refractory mended by a method where a material obtained by mixing water and a blowing material in advance so as to be a slurry is blown from a tank, that is, a wet blowing method.
As the thermal spray material to be used in such a flame spray mending method, for example, a highly siliceous thermal spray material containing 93.9 to 99.6% by weight or more of SiO.sub.2, 1.5% by weight or less of Al.sub.2 O.sub.3, 2.0% by weight or less of CaO, 1.0% by weight or less of Fe.sub.2 O.sub.3, and 0.4 to 2.0% by weight of Na.sub.2 O is proposed in the official gazette of Japanese Examined Patent Publication No. 3-9185. In general, this kind of material is a material having a 60% or more crystallization ratio immediately after thermal spraying where crack generation according to the expansion at the time of the crystallization of the amorphous (vitreous) part (&lt;40%), and decline of the adhesion strength caused by the difference in the heat expansion characteristics between the thermal spray mending layer and the coke oven wall bricks are observed. That is, the material according to the above-mentioned proposal has been developed in order to overcome the problem derived from the low crystallization ratio.
However, the technology disclosed in the official gazette of Japanese Examined Patent Publication No. 3-9185 has a problem in that the thermal spray condition for having a thermal spray mending layer with a 60% or more crystallization ratio in the material, that is, the oxygen gas flow rate, and the propane gas flow rate is limited in an extremely narrow range. Furthermore, with the thermal spray condition capable of obtaining a thermal spray mending layer with a 60% or more crystallization ratio, a dense thermal spray mending layer, that is, a thermal spray mending layer having a high compression strength cannot be obtained easily, and thus a problem is involved in that the wear resistance is poor and the life of the thermal spray mending layer is short.
Moreover, as the SiO.sub.2 material, which is the main component of the conventional thermal spray mending material, silica brick scrap is used frequently for reduction of the cost. However, when the brick scrap is used as the material, a lot of impurities are introduced. In particular, since CaO is a substance broadly present as a binder in silica brick production, CaO is introduced inevitably and thus it is difficult to limit the amount of CaO component to 2% by weight or less. Besides, since CaO has a strong effect of lowering the crystallization ratio immediately after thermal spraying in a SiO.sub.2 thermal spray coat layer, the crystallization ratio needs to be improved by adjusting the other components when the CaO component is present in a large amount.
As heretofore explained, problems still remained for the conventional technology include tendency of crack generation in the mended layer and a low adhesion strength with respect to the base material surface. It has problems at least in that the need for improving the crystallization ratio is severe and the compression strength cannot be improved so that the wear resistance is poor and the wall life is short.
In order to improve the product crystallization ratio immediately after thermal spraying of the flame spray mending material mainly containing SiO2, it is of course effective to eliminate a component disturbing the crystallization, but there is a limitation for the use of a highly pure material in view of the high material cost. For that reason, conventionally, silica brick scrap has been reused in most cases as the SiO.sub.2 material. On the other hand, as a flame spray mending material, one having an 80% or more crystallization ratio immediately after thermal spraying, even in a condition where CaO is inevitably introduced from the silica brick scrap, and satisfying a 200 kgf/cm.sup.2 compression strength, is required for mending a coke oven wall brick.
Accordingly, an object of the present invention is to provide a thermal spray mending material having a high crystallization ratio immediately after thermal spraying and effective in dealing with a dense thermal spray mending layer in a broad thermal spray condition. Moreover, another object of the present invention is to provide a thermal spray mending material having excellent wear resistance and durability (life) by ensuring a high compression strength on one hand without the risk of a mending layer crack or a decline in the adhesion strength with respect to the mending surface.
Still another object of the present invention is to obtain a thermal spray material capable of producing a thermal spray layer having an 80% or more crystallization ratio immediately after thermal spraying and a high compression strength (&gt;200 kgf/cm.sup.2) even when CaO is inevitably introduced in silica brick scrap to some extent.